test(mcp): add strict live schema codec

[barbatdev]

🔗 Linked Issue

Upstream issue: Gentleman-Programming#717 (status:approved).

This is a fork-first staging PR and intentionally does not close the upstream issue.

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🏷️ PR Type

• type:bug — Bug fix
• type:feature — New feature
• type:docs — Documentation only
• type:refactor — Code refactoring (no behavior change)
• type:chore — Maintenance, dependencies, tooling
• type:breaking-change — Breaking change

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📝 Summary

• Add a strict test-only codec for the live MCP tool input-schema contract.
• Normalize the bounded v1 vocabulary with deterministic JSON Pointer diagnostics.
• Reject duplicate JSON members, trailing values, unsupported facts, and lossy number handling.

📂 Changes

File	Change
internal/mcp/tool_contract_test.go	Adds fixture-independent schema decoding, normalization, live-registry observation, and focused tests.

🧪 Test Plan

• Focused MCP schema-codec tests pass.
• go test ./internal/mcp -count=1 passes with 88.8% package coverage.
• go build ./... passes.
• Fixture-isolated focused and package tests pass.
• go test ./... is fully green locally: two known macOS /var versus /private/var assertions fail outside internal/mcp.

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🤖 Automated Checks

Fork-first note: the upstream issue is approved, but the fork-local governance job may not resolve an upstream issue through context.repo. The final upstream PR will use the canonical issue-closing reference.

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✅ Contributor Checklist

• The approved upstream issue is linked above.
• Exactly one PR type is selected.
• A matching type:feature label is requested.
• Commits follow conventional-commit format.
• No Co-Authored-By trailers are present.
• No runtime behavior or fixture dependency is introduced.

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💬 Notes for Reviewers

Fork-first stack, PR 1 of 3. This slice is intentionally fixture-independent and changes one file with 329 additions. PR 2 adds the versioned fixture and exact guard; PR 3 adds directional compatibility, the guarded writer, and inventory de-duplication.